1. Field of the Invention
The present invention concerns a method for switching regional saturation pulses in magnetic resonance spectroscopy and magnetic resonance imaging. The present invention is particularly applicable where signal portions that are of interest in the examination could also be saturated in addition to the saturated signal upon the application of a saturation pulse.
2. Description of the Prior Art
Localized (targeted) saturation pulses are used for suppression of unwanted signals in magnetic resonance spectroscopy. The spatial extent of the saturation range can be established by the application of a magnetic field gradient and by selection of the frequency of the saturation pulse. Dependent on the spatial relation of these saturation pulses relative to the examination region (volume of interest, VOI), the signal portions of compounds that are actually of interest in the examination, but are partially or completely suppressed in the data acquisition due to the switching of the saturation pulses, can also be suppressed due to the chemical shift artifact.
The phenomenon known as the chemical shift is the property utilized in spectroscopy that the resonance frequency shifts slightly (proportional to the magnetic field strength) dependent on the type of chemical compound in which the precessing nucleus is located. Due to the switching of magnetic field gradients during the saturation pulses, the position of the saturated region depends on the chemical shift. For example, for a saturation pulse with a predetermined bandwidth, the position of the saturated water portions differs from the position of the saturated fat portions since a difference in the resonant frequency of approximately 3.7 ppm (parts per million) exists between fat and water.
Due to the different position of the saturation region for various chemical compounds, it can now occur that signals of nuclei of interest in the examination region can also be suppressed given selection of a saturation region in addition to the examination region. This effect increases with the chemical shift and is particularly pronounced when long saturation pulses or pulses with small bandwidths must be used, which occurs with the use of high magnetic fields and excitation with large coils. This unwanted saturation makes the evaluation of the data in the affected regions impossible.
EP 1 022 576 A1 describes a method for fat suppression in which the fat signal is suppressed by a CHESS pulse sequence, wherein the optimal flip angle is calculated.
U.S. Pat. No. 6,304,084 discloses the use of 180° pulses after a 90° pulse in order to reduce the chemical shift artifact.
It has been previously attempted to minimize this problem by using saturation pulses with high bandwidths and rectangular slice profiles. A disadvantage of this approach is that very high voltages, the generation of which is costly, are required for generation of such RF pulses.